It is well known that impact strength of styrene resins which are hard and brittle can be improved by blending a styrene resin with an elastomer or polymerizing a styrene monomer in the presence of an elastomer to obtain a rubber-modified styrene resin composition. However, the rubber-modified styrene resin composition is still inferior to ABS resin in impact strength.
To meet the recent demands in market for decreased cost and reduced wall thickness, it has been keenly desired to develop a rubber-modified styrene resin composition possessing impact strength equal to ABS resin.
Methods for improving impact strength of a rubber-modified styrene resin composition have so far been proposed as described, e g., in JP-A-57-172948, JR-A-57-187345, A-57-187345, JP-A-57-187346, JP-A-60-166338, JP-A-61-85461, JP-A-49-354, and JP-A-59-66412 (the term "JP-A" as used herein means an "unexamined published Japanese Patent application"). Each of these methods comprises addition of an organopolysiloxane as an impact modifier or copolymerization of monomers such as acrylonitrile and .alpha.-methylstyrene. However, such techniques involve a disadvantage of high cost incurred for purchase of an additive, installation of feeders, storage of monomers, or compounding equipments. Besides, the degrees of improvement on impact strength attained by these methods are still unsatisfactory.
Furthermore, in order to improve gloss, it is necessary to render the particle size of soft component particles comprising an elastomer and a styrene resin graft polymerized on an elastomer and occluded styrene polymer as small as possible to an extent such that it is corresponding to a wavelength of light. However, if soft component particles having a small particle size is used, the impact strength decreases. Accordingly, in the case that in order to improve the gloss, the particle size of the soft component is rendered small, though the impact strength can be generally improved by increasing the amount of the elastomer added, there is generated a defect that an increase in amount of the elastomer added leads to a reduction in stiffness.